manual: Improve documentation of get_current_dir_name. [BZ #6889]

This is a minor rewording to clarify the behaviour of
get_current_dir_name.  Additionally, the @vindex is moved above the
@deftypefun so that following links give a better result with regard
to context.

	[BZ #6889]
	* manual/filesys.texi (get_current_dir_name): Clarify
	behaviour.